This invention relates generally to zone-refining of semiconductor materials and more particularly to a method and apparatus for automated and precise control of the refining and growth of monocrystalline silicon rods.
Present manufacture of semiconductor materials requires a high degree of purity in the silicon used in making the individual components. Suitably high material purities have been achieved in a process known as zone-refining which utilizes the phenomenon where impurity concentrations in a freezing solid will always be less than the concentration in the non-frozen liquid. Consequently, if a semiconductor rod is melted completely across its cross-section and that melted zone is moved longitudinally along the rod, the melted zone will have a much higher concentration of impurities than the solid or frozen zone. Making several passes along the bar with a heating source, which melts the bar only in the immediate vicinity of the heating source, will provide a high degree of purity in the bar with the exception of the end towards which the liquid phase is moved, which will contain the majority of the impurities previously distributed throughout the bar. A prerequisite of zone refining is that the container material for the bar must not react with the semiconductor material, otherwise impurities may be added to the liquid semiconductor material. In the case of some semiconductor materials, such as silicon, which react with most known container materials, a process called floating-zone refining has been developed.
In the floating-zone refining process, a molten zone of liquid silicon is suspended by its high surface tension between two colinear vertical rods of silicon. The liquid zone is moved by the relative movement of the heating source, which normally takes the form of a radio frequency (RF) inducation heating coil. Either the coil may be moved along the length of the rod or, preferably, the rod is moved through the coil. As the rod is moved down through the coil, the portion of the rod within the coil is melted, and the liquid zone then refreezes into the bottom portion of the rod as the molten portion moves upward. Normally, this process is initiated by a small diameter seed crystal on the bottom of the rod, and then the molten zone is slowly moved upward and away from this seed such that a monocrystalline structure is achieved and maintained in the refreezing bottom portion of the rod. It is advantageous to have refined rods of a larger diameter than the seed crystal where the seed crystal diameter is on the order of 5 millimeters in diameter and the finished rod is on the order of 100 millimeters in diameter. The larger diameter rods are later cut into wafers, and since they must be machined to a precise diameter, it is necessary to grow a rod only slightly larger than this diameter initially. To avoid waste of the semiconductor material, it is advantageous to grow the rod as close as possible to the desired diameter, and therefore precise diameter control is desired.
To go from the 5 millimeter to the 100 millimeter diameter requires very precise control of the melting area, such control being achieved by varying the heating rate, the relative movement between the rod portions as well as the rotational speed of the rod portions to achieve uniform heating throughout the cross-sectional area. This tapering process is extremely critical because the molten silicon must bulge or "bag" over the edge of the frozen lower rod portion so that the lower rod diameter continues to increase in an outward taper. In the past this has only been accomplished with a skilled human operator at the various controls to assure that the quantity of molten silicon is not so great as to cause a "spillover" of liquid where the molten silicon spills down the side of the newly frozen rod. The prior art indicates methods of diameter control which control the relative movement between rod portions and/or the heating rate of the induction coil based upon the freezing diameter of the straight rod only. These methods are not adaptable to controlling the growing operation during taper and are only automatic after the final rod diameter has been reached. Therefore, the tapering process is totally dependent upon the skill of the human operator from start to the end of the taper thus being subject to human error and failings. It is desirable therefore to eliminate the need for human operators and to be able to accomplish the starting and ending taper, the transistion from tapering growth to straight and back to taper, as well as maintain close tolerances on the sides of the straight portion of the crystal, completely automatically.
In the past, various methods of optically sensing and controlling the diameter of a straight crystal rod during refining have been used. See for example, Keller, U.S. Pat. No. 2,992,311. Unfortunately, none of the previous methods have provided an automated means to initiate the growth from the seed crystal diameter to the finished rod diameter and then back to a small diameter, necessitating the use of a highly skilled human operator at least during the tapering phase of the crystal growth. When existing processes were modified in an attempt to automate this growth process, it was found that using the diameter of the refrozen rod as a sensing base had too much lag time between the sensing of a change and the response signal to the zone refiner controls. Therefore, the crystals either "pull apart", breaking the continuity between the melting crystal and the freezing crystal rods, or else they "freeze out" indicating that the liquid zone freezes prematurely terminating the refining process.